The field to which our invention relates is that known as electrical discharge machining, hereinafter sometimes referred to as "EDM," in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges between one or more tool electrodes and the workpiece. A servo-feed system is employed to provide relative movement and thus to maintain an optimum gap spacing between electrode and workpiece as material is progressively removed from the workpiece. A dielectric coolant fluid is circulated and recirculated, generally under pressure, to the gap throughout the machining operation. For most reliable and dependable results, a power supply of the independent pulse generator type is utilized to provide machining power pulses of precisely controlled frequency and on-off times.
During EDM machining operation, it is sometimes necessary to change the gap polarity in accordance with the selection being made of electrode material and changeover being made with respect to the workpieces being machined. For example, in electrical discharge machining of some workpiece materials, it is desirable to use copper electrodes in which polarity will be "standard," with the electrode being negative and the workpiece being positive. For other workpiece materials, it is preferred to use graphite electrodes in which case the polarity will be that commonly referred to as "reverse," when the electrode is positive and the workpiece negative. In any machining power supply having the capability of polarity changeover through relay or other similar switching arrangement, it is essential that the output transistors be triggered both on and off with a clean, square wave triggering pulse. This is accomplished by generating and applying two out-of-phase signals from the multivibrator.